In composite structures of this type, each ply has a ply orientation defined by principal stiffness direction of the ply. For example the structure may have a lay-up with primary plies, cross plies with a ply orientation substantially perpendicular to the primary plies, positive-angled bias plies with a ply orientation at a positive acute angle to the primary plies, and negative-angled bias plies with a ply orientation substantially perpendicular to the positive-angled bias plies. Typically the ply orientation of the primary plies is more closely aligned with a principal bending axis of the structure than the cross plies or the bias plies. It is known to vary the lay-up in order to achieve desired physical characteristics. For instance the proportion of primary plies may be increased in order to increase the stiffness of the structure along its principal bending axis. In such a situation the primary plies and cross plies are said to be “un-balanced”. Typically the number of positive-angled and negative-angled bias plies is equal in order to prevent undesired bending torsion coupling which can lead to distortion of the part after curing due to the mismatch in thermal expansion between the positive and negative-angle bias plies. In such a situation the bias plies are said to be “balanced”.